NMR or MRI
In Magnetic Resonance Imaging (MRI) systems and Nuclear Magnetic Resonance (NMR) systems, a static magnetic field (B) is applied to a body under investigation. The static magnetic field defines an equilibrium axis of magnetic alignment in a region of the body under investigation. An RF field is applied in the region being examined in a direction orthogonal to the static field direction. The RF field excites magnetic resonance in the region, and resulting RF signals are detected and processed. Generally, the resulting RF signals are detected by RF coil arrangements placed close to the body. See for example, U.S. Pat. No. 4,411,270 to Damadian and U.S. Pat. No. 4,793,356 to Misic et al. Typically, these coils are either surface type or volume type coils, and, depending on the application, are used to transmit RF and receive NMR signals from the region of interest (ROI).
Signals pertaining to different nuclei exhibiting ±½ spin can be observed using NMR. While proton (1H) is used commonly for MRI, signals from other nuclei (31P, 13C, 23Na, 19F, 7Li, etc.) are used to obtain bio-chemical and other information (e.g., functional, physiological, vascular etc.) from the object under investigation. The sensitivities of these nuclei, however, are much smaller than water protons. Hence coils with improved signal-to-noise ratio (S/N) are sought to improve the quality of the MR data obtained with these nuclei. Other techniques, such as proton decoupling and inverse experiments, are utilized to enhance the sensitivity of the NMR experiment. Thus, the dual tune coil chosen for the experiment must be capable of simultaneous operation. This should be done without sacrificing the performance of the low gamma nuclei.
Birdcage Coil
The birdcage coil is well known in the art (see, e.g., U.S. Pat. No. 4,672,705 issued to Hayes, the entire disclosure of which is incorporated by reference) and includes two end rings connected by several straight segments, which are referred to as legs. The birdcage coil has several resonance modes, of interest being the principal k=1 mode for homogeneous imaging. The principal mode has two linear modes oriented orthogonal to one another. The outputs from these modes can be combined using analog circuitry or digitally combined in a receiver system. The birdcage provides about a 41% improvement in S/N and expends about one-half of the power of conventional linear coils.
In addition, owing to the sinusoidal currents in the coil periphery, the birdcage provides a highly homogeneous B field in the transverse planes (XY) inside the coil, which is ideal for imaging whole-body, head, knee, wrist, etc. for adults. The B field profile along the coil axis, however, mimicks a gaussian distribution with maximum at the coil center.
The B field distribution can be improved over the adult head with an end-capped design of Hayes (see, e.g., Book of Abstracts, 5th ISMRM, p39–40), which provides a more uniform B field distribution toward the top of the head. At the open end of the coil axis, the B field distribution for the end-capped coil design falls off like a conventional birdcage, which is ideal for imaging the adult head.
A dual-tuned birdcage (U.S. Pat. No. 4,799,016 issued to Rezvani et al.) used two birdcages in a lapped concentric fashion (one inside the other). Note, the low frequency birdcage coil located inside the high frequency birdcage coil will prevent flux from the outer high frequency birdcage coil from penetrating the dual-tuned coil's volume. The outer high frequency coil will be capacitive and let all of the flux from the low frequency coil pass through the dual-tuned coil's volume with little or no effect. Thus, the efficiency of the high frequency coil (proton coil) is reduced, which was seen by Fittzsimmons et al. (see e.g., Double Resonance Quadrature Birdcage, Mag Res Med, 1993, 30:107–114).
Likewise, if the low frequency coil is located outside the high frequency coil, flux from both coils penetrates the dual-tuned coil volume (over the imaging field of view [FOV]). But since the low frequency coil is located outside of the high frequency coil, the filling factor and hence the resultant S/N is low.
These effects prompt in favor of a design where the performance of a coil at the NMR frequencies can be maintained despite the dual frequency quadrature operation or despite the presence of a second coil near it.
A novel four-ring design was introduced in U.S. Pat. No. 5,194,811 issued to Murphy-Boesch et al., but here the FOV of the low gamma nucleus is much shorter than protons, which is undesirable if the coverage over the entire brain is sought.
A new design is solicited that will provide high signals and a high degree of RF homogeneity at both the NMR frequencies over the imaging FOV.